Development of a displacement sensor for the CERN-LHC superconducting cryodipoles

One of the main challenges of the Large Hadron Collider (LHC), a new partic le accelerator currently under construction at CERN (the European Organizat ion for Nuclear Research) in Geneva, resides in the design and production o f the superconducting dipoles used to steer the particles around a 27 km un derground tunnel. These so-called cryodipoles consist of an evacuated cryos tat and a cold mass containing the particle tubes and the superconducting d ipole magnet. The latter is cooled by superfluid helium at 1.9 K. The parti cle beams must be centred in the dipole magnetic field with a sub-millimetr e accuracy. This requires that the relative displacements between the cryos tat and the cold mass must be monitored with great accuracy. Because of the extreme environmental conditions (the displacement measureme nts must be made in vacuum and between two points at a temperature differen ce of about 300 degrees) no adequate existing monitoring system was found f or this application. It was therefore decided to develop an optical sensor based on low-coherence double interferometry, which measures with micromete r precision the distance between a mirror welded to the dipole cold mass an d an optical head attached in the inner wall of the cryostat. This contribution describes the development of this novel sensor and the fi rst measurements performed on the LHC cryodipoles.